1. Field of the Invention
The present invention relates to radio apparatus, and it particularly relates to a radio apparatus using multiple subcarriers.
2. Description of the Related Art
An OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme is one of multicarrier communication schemes that can realize the high-speed data transmission and are robust in the multipath environment. This OFDM modulation scheme has been applied to the wireless LAN (Local Area Network) standards such as IEEE802.11a/g and HIPERLAN/2. The packet signals in such a wireless LAN are generally transferred via a time-varying channel environment and are also subject to the effect of frequency selective fading. Hence, a receiving apparatus generally carries out the channel estimation dynamically.
In order for the receiving apparatus to carry out the channel estimation, two kinds of known signals are provided within a packet signal. One is the known signal, provided for all carries in the beginning of the burst signal, which is the so-called preamble or training signal. The other one is the known signal, provided for part of carriers in the data area of the packet signal, which is the so-called pilot signal (See Reference (1) in the following Related Art List, for instance).
Related Art List
    (1) Sinem Coleri, Mustafa Ergen, Anuj Puri and Ahmad Bahai, “Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems”, IEEE Transactions on broadcasting, vol. 48, No. 3, pp. 223-229, September 2002.
In wireless communications, adaptive array antenna technology is one of the technologies to realize the effective utilization of frequency resources. In adaptive array antenna technology, the directional patterns of antennas are controlled by controlling the amplitude and phase of signals, to be processed, in a plurality of antennas, respectively. One of techniques to realize higher data transmission rates by using such an adaptive array antenna technology is the MIMO (Multiple-Input Multiple-Output) system. In this MIMO system, a transmitting apparatus and a receiving apparatus are each equipped with a plurality of antennas, and packet signals to be transmitted in parallel are set (hereinafter, each of data to be transmitted in parallel in a packet signal is called “stream”). That is, streams up to the maximum number of antennas are set for the communications between the transmitting apparatus and the receiving apparatus so as to improve the data transmission rates.
Moreover, combining this MIMO system with the OFDM modulation scheme results in a higher data transmission rate. In the MIMO system, the data rate can also be adjusted by increasing or decreasing the number of antennas to be used for data communications. Furthermore, the data rate can be adjusted in greater detail by applying the adaptive modulation to the MIMO system. To perform such an adjustment of data rates more reliably, the transmitting apparatus should acquire from the receiving apparatus the information on data rates suited for the radio channel between the transmitting apparatus and the receiving apparatus (hereinafter referred to as “rate information”). To raise the accuracy of the rate information like this, it is desired that the channel characteristics between a plurality of antennas in the transmitting apparatus and a plurality of antennas contained in the receiving apparatus, respectively, be acquired by the receiving apparatus.
Examples of the combinations of directivity patterns in the antennas of the transmitting apparatus and receiving apparatus in a MIMO system are as follows. One example is a case where the antennas of a transmitting apparatus have omni patterns and the antennas of a receiving apparatus have patterns in adaptive array signal processing. Another example is a case where both the antennas of the transmitting apparatus and those of the receiving apparatus have patterns in adaptive array signal processing. This is also called the beamforming. The system can be simplified in the former case. In the latter case, however, the directivity patterns of antennas can be controlled in greater detail, so that the characteristics thereof can be improved. Since in the latter case the transmitting apparatus performs adaptive array signal processing for transmission, it is necessary to receive beforehand from the receiving apparatus the known signals by which to estimate channels.
To improve the accuracy of rate information and the accuracy of beamforming in the above-mentioned requirements, it is necessary that the channel characteristics be acquired with high accuracy. To improve the accuracy in the acquisition of channel characteristics, it is desirable that the channel characteristics between a plurality of antennas contained in the transmitting apparatus and those in the receiving apparatus be acquired respectively. For this reason, the transmitting apparatus or the receiving apparatus transmits from all of antennas the known signals for use in channel estimation. Hereinafter, the known signals, for use in channel estimation, transmitted from a plurality of antennas will be referred to as “training signals” independently of the number of antennas to be used for data communication.
Under these circumstances, the inventor of the present invention came to recognize the following problems to be solved. When the training signals are transmitted, the number of streams containing known signals for use in channel estimation (hereinafter referred to as “channel estimation known signals”) differs from that containing data. A known signal for setting AGC (Automatic Gain Control), hereinafter referred to as “AGC known signal”, at the receiving side is assigned anterior to the channel estimation known signals. When an AGC known signal is assigned only in a stream where data is assigned, one of the channel estimation known signals is received in a state where the AGC known signal has not been received anterior thereto. In particular, when the strength of AGC known signal gets smaller at the receiving side, the gain of AGC is set to a value which is large to a certain degree. In so doing, when the strength of channel estimation known signal of a stream where the AGC known signal is not assigned is larger, there is a strong possibility that said channel estimation known signal may be amplified to such a degree that distortion is caused by AGC. As a result thereof, the error in channel estimation based on said channel estimation known signal becomes large.
On the other hand, when an AGC known signal is assigned to a stream where a channel estimation known signal is assigned, the number of streams to which the AGC known signal is assigned differs from that to which data is assigned. Hence, there is a possibility that the gain set by the AGC known signal is not suitable for the modulation of data. As a result, the demodulated data are subject to errors.